1. Field of the Invention
The present invention relates to a colpitts quadrature voltage controlled oscillator, and more particularly, to a colpitts quadrature voltage controlled oscillator that is capable of obtaining a precise quadrature orthogonal signal without using a separate coupled device or a passive/active device, has low phase noise, consumes less electric power, and is compact in size.
2. Discussion of Related Art
Currently, telecommunication transceiver systems are switching from a super-heterodyne architecture to a direct conversion architecture, which has several advantages in terms of circuit structure, power consumption, and cost.
In a direct conversion transceiver, orthogonal signals having the same amplitude and having phases delayed by 90 degrees are required for signal conversion. Such orthogonal signals are generated using a colpitts quadrature voltage controlled oscillator.
FIG. 1a is a circuit diagram of a conventional colpitts differential voltage controlled oscillator 100. The voltage controlled oscillator 100 of FIG. 1a is a capacitance-feedback common-collector colpitts differential voltage controlled oscillator and has a balanced structure that generates signals that are 180 degrees out of phase.
As illustrated in FIG. 1a, the conventional colpitts differential voltage controlled oscillator 100 includes a first LC resonator 101 connected to a power supply voltage Vcc and generating a resonance frequency, first and second oscillators 102 and 103 including first and second oscillation transistors Q1 and Q2 that oscillate at the resonance frequency to output an oscillation signal, feedback capacitors Cf and a degeneration capacitor Ce connected between the first and second oscillation transistors Q1 and Q2, and current sources Is.
In the first oscillator 102 and the second oscillator 103, the feedback capacitors Cf are respectively connected between bases and emitters of the first and second oscillation transistors Q1 and Q2, and the degeneration capacitor Ce is connected between the emitter of the first oscillation transistor Q1 and the emitter of the second oscillation transistor Q2. The current sources Is are respectively connected between the emitters and ground of the first and second oscillation transistors Q1 and Q2 and the power supply voltage Vcc is connected to collectors of the first and second oscillation transistors Q1 and Q2.
A first output node I+ of the first LC resonator 101 is connected to the base of the first oscillation transistor Q1, a second output node I− is connected to the base of the second oscillation transistor Q2 to form a differential oscillator structure, and differential signals that are 180 degrees out of phase are generated from the first output node I+ and the second output node I−.
Briefly describing operation of the colpitts voltage controlled oscillator 100, first, values of the feedback capacitors Cf and the degeneration capacitor Ce are adjusted to generate a negative resistance. Here, the negative resistance may be generated in a desired frequency band by adjusting the values of the feedback capacitors Cf and the degeneration capacitor Ce.
As described above, the values of the feedback capacitors Cf and the degeneration capacitor Ce are adjusted to generate the negative resistance in the desired frequency band, so that the first LC resonator 101 can resonate at a desired frequency.
The resonance frequency generated by the first LC resonator 101 is input to the bases of the first and second oscillation transistors Q1 and Q2 to cause oscillation.
FIG. 1b illustrates a differential signal generated by the colpitts differential voltage controlled oscillator 100 of FIG. 1a. As illustrated in FIG. 1b, differential signals that are 180 degrees out of phase are generated from the first output node I+ and the second output node I− of the first LC resonator 101.
While quadrature orthogonal signals having the same amplitude and having phases delayed by 90 degrees are required in the direct conversion transceiver, according to the conventional colpitts differential voltage controlled oscillator 100, only differential signals that are 180 degrees out of phase are obtained, as illustrated in FIG. 1b. The following three (3) methods of obtaining quadrature orthogonal signals have been disclosed.
A first method involves using a coupled transistor or a coupled transformer to combine two voltage controlled oscillators so that a quadrature orthogonal signal is obtained. However, this method results in unnecessary nonlinearity, increased phase noise, decrease in the Q-factor of an LC resonator, and increased power consumption due to the coupled transistor or the coupled transformer.
A second method involves connecting a frequency divider to the rear end of a differential voltage controlled oscillator. In this case, a differential voltage controlled oscillator that runs at twice the desired frequency is required, which results in increased phase noise and electric power consumption.
A third method involves connecting a multiphase RC filter to the rear end of a differential voltage controlled oscillator. In this case, it is difficult to obtain a precise orthogonal signal, and thus a separate phase calibration circuit is required. Consequently, the oscillator circuit becomes complex and electric power consumption increases.